In integrated circuits (ICs), noise generated in one part of the chip by one circuit may couple into circuits in other parts of the chip by propagating through the common substrate. In mixed signal integrated circuits where sensitive analog circuits operating in micro-volt range reside on the same substrate as high speed digital circuits switching between zero and V.sub.DD, the substrate coupled noise can adversely disrupt the performance of the analog circuits.
Mixed signal ICs that are plagued by this problem include data converters, hard disk drive products, and integrated DSP and baseband chips. Because the noise is capacitively coupled, this problem becomes even more severe at high frequencies, such as in radio frequency (RF) circuits used in cellular phones, wireless local area networks (LANs), personal communication service (PCS) products, and remote control RF products.
The problem of substrate coupled noise currently has no good solution. The integrated circuit engineers and manufacturers are forced to put analog circuits and digital circuits in separate integrated circuits or use extreme care to separate analog and digital circuit blocks and use multiple guard rings around the sensitive circuits. A guard ring is typically a p+ (or n+) diffusion or an n-well (or p-well) structure which circle the sensitive circuit. The p+ diffusion guard ring is not very effective in insulating the sensitive circuits from substrate coupled noise due to the inductance of the bond wire connecting the guard ring to ground. The bond wire has some resistance at high frequencies, which inhibits a total collection and evacuation of the noise carriers. The n-well guard ring is also not very effective because it does not collect the noise carriers but merely diverts them a little deeper into the substrate, where they may still adversely affect the sensitive circuits. Without a viable solution, substrate coupled noise currently limits the performance of high speed, high accuracy mixed signal ICs. Further, highly integrated RF chip implementation is not possible without substantially reducing the substrate noise coupling.